This error can occur in PowerUser from the results of a search for document or memo content. If you try to open a different vault that does not have the Enable context indexing for documents and memos option enabled, PowerUser tries to run the same full-text search in the vault that you are trying to open and the search fails because there is no full-text index for that vault. To open the other vault without error, first either change the search criteria to exclude document or memo content or open a navigation view.